Many types of image forming devices pick a media sheet from a storage location and move the media sheet to an imaging location for receipt of a toner image. The timing of the media sheet relative to the imaging location is important for adequate toner image receipt and image formation. Improper timing results in top writing line margin error with the toner image positioned at the wrong location relative to the top margin of the media sheet.
Expected time allocations are used to determine the timings for picking a media sheet from an input tray such that it reaches a transfer point to receive the toner image. Deviations from the expected times require additional demand on the system and may result in inadequate image formation.
One deviation in the expected time allocations is caused by the friction of the pick mechanism as the media sheet leaves the input tray. The pick mechanism contacts the media sheet at the input tray and transports the sheet a distance where it is introduced and driven by the paper path. At the introduction point into the paper path, the media sheet may still be in contact with the pick mechanism. The pick mechanism may impede the movement of the sheet by the paper path resulting in the sheet moving slower than expected and thus deviating from the expected time.
The Model Z65 printer available from Lexmark International, Inc. uses a ball-clutch design for picking media sheets from an input tray. The Z65 ball-clutch includes a one ball—two pocket design which reduces or prevents friction on the media sheet when controlled by two separate sections of the paper path. However, the ball-clutch causes deviations in the amount of time necessary to pick the media sheet from the input tray. The Z65 printer is able to use a ball clutch because image transfer on Z65 does not occur until the media sheet is in the proper position (i.e., the media sheet reaches the transfer point prior to the imaging). Therefore, pick timings for Z65 printer are not as critical and deviations of the one ball—two pocket design can be accounted for. Serial printers which feature toner image formation on an intermediate mechanism which intersect a media sheet at a transfer point require more critical timing because the imaging operation may start before the media reaches any sensors in the paper path. Any variation in the pick timings translate into top writing line margin error that should be corrected by the printer before the media sheet reaches the transfer point. Only a finite amount of error can be corrected.